A "SCRAM" button for my desktop.
Stops Banshee or iTunes from playing music (if it's running), and starts my screensaver.
Useful for grabbing a cup of coffee, or as a ZOMG-THE-BOSS-IS-BEHIND-ME button.
Uses a USB serial adapter and physical button to pull the DSR line to Ground to trigger.
I also remarked about this on my blog: http://www.mibus.org/2012/02/22/desktop-scram/